1. Field of the Invention
The present invention relates to a system for monitoring power of a nuclear reactor and a power distribution thereof.
In particular, the present invention relates to the system for monitoring the power of the nuclear reactor, which is used for monitoring an oscillation of a power of a boiling water reactor.
2. Description of the Prior Art
A boiling water reactor (hereinafter, referred simply to as BWR) such as a light water reactor is provided with a reactor core instrumentation system having an arrangement in which multiple neutron flux detection devices, as plural neutron measuring means, is disposed in the core for detecting neutron flux so that the nuclear instrumentation system monitors power of the reactor and power distribution in axial and radius direction of the reactor core according to the detected neutron flux in an operational state of the BWR.
One neutron flux detection device is arranged in each of the 16 fuel assemblies in the reactor core of the BWR. Each neutron flux detection device has four neutron flux detectors disposed along a vertical direction thereof. Each of the neutron flux detectors are called as a local power range monitor (LPRM). For example, in a reactor core of a 1100 MWe BWR, 172 (=43xc3x974) channel neutron flux detectors are disposed.
A power signal (LPRM signal) from each neutron flux detector is averaged every about 20 signals by an average power range monitor (APRM). For example, the 1100 Mwe BWR has 6 channels of the average power range monitors so that 6-channel APRM signals are outputted therefrom. All of these APRM and LPRM signals are analog signal.
In the conventional BWR, the APRM signal from the average power range monitor is monitored, and when the APRM signal gets to reach to no less than a predetermined value (predetermined point), a trip signal such as a scram signal is outputted so that the nuclear reactor is not prevented from operating in a dangerous state in response to the trip signal, and is stably operated.
In particular, in the conventional BWR, for avoiding an unstable phenomenon in a power of a reactor core, an operating limit range is previously set so as to avoid an operation of the nuclear reactor in the operating limit range. In a case where the reactor is operated in the operating limit range, the measures, in which a previously selected control rod is inserted so as to lower the power of the nuclear reactor whereby the operation of the nuclear reactor gets away from the operating limit range, are taken.
The operating limit range of the nuclear reactor is previously computed from a result of stability analysis using an analysis code by a process control computer. Recently, there has been developed a stability monitor which can continuously evaluate a stability of the power of the reactor core on the basis of vacillating of the signals (LPRM signals) in the reactor core detected by the neutron flux detector.
However, since the conventional stability monitor makes only an evaluation of stability with the use of the averaged APRM signal and an evaluation of individual LPRM signals, it is impossible to accurately make an evaluation of stability in a power oscillation of the nuclear reactor. The stability in the power oscillation of the nuclear reactor depends upon a complicate space dependency in the reactor core, and is recently observed in many foreign atomic power plants.
Since the APRM signal in averaged equally by each of the LPRM signals, in case where the power of the nuclear reactor oscillates in the whole of the reactor core, it is possible to detect a power distribution of the reactor core. However, in a case where the power of the nuclear reactor locally oscillates in the reactor core, or in a case where the power thereof oscillates while having a spatial phase difference in the reactor core, each of quantities of the oscillations included in each of the LPRM signals is offset so as to be absorbed in each other due to averaging the LPRM signals, thus, there is the possibility that it is difficult to detect the power distribution of the reactor core.
As an example of the oscillation of the power of the nuclear reactor at local areas in the reactor core, there is an oscillation phenomenon of a density wavy oscillation generated from fuel assemblies which are thermal-hydraulically strict and have high-power, respectively, said oscillation phenomenon being called as a channel oscillation phenomenon. Even if the oscillation phenomenon is diffused by a neutron flux oscillation, there is the possibility that the oscillation phenomenon is adapted to oscillate in a relatively only narrow range.
Further, as an example of causing the oscillation of the nuclear reactor having the spatial phase difference, there is an oscillation phenomenon which is called as a regional oscillation such that the power signals are mutually oscillated with a 180xc2x0 phase difference at a symmetrical position in the reactor core. This oscillation phenomenon is actually observed in some foreign atomic power plants. For example, in a regional oscillation observed in a CAORSO plant in Italy, the largest amplitude of the APRM signal is, at most, a degree of approximately 10%; on the contrary, the largest amplitude of the LPRM signal reaches 60% is observed. This results from the following reason in which the power signals corresponding to the LPRM signals are mutually oscillated (vibrated) with a 180xc2x0 phase difference at each half portion of the reactor core so that maximum values of the LPRM signals and minimum values thereof are simultaneously averaged so as to be canceled.
In a case of monitoring a stability of the reactor core, usually, a damped wave ratio indicative of stability, a cycle of the oscillation, an amplitude thereof and the like are computed from the APRM signal, whereby the stability of a state of the reactor core is estimated.
However, in the case where the regional oscillation is generated, even if the only APRM signal is monitored, there is the possibility that it is impossible to accurately detect the stability of the reactor core.
Further, in a system for monitoring a stability of a nuclear reactor, some LPRM signals at different portions in the reactor core are selected so that, by using the selected LPRM signals, the estimation operation of the stability of a state of the reactor core is carried out in the same manner as the APRM signal. However, since a logic for processing a plurality of LPRM signals and for carrying out a decision operation is not decided, the estimation operation of the system is not used for detecting the stability of the reactor core in a case where the regional oscillation is generated.
The applicants of the present invention have proposed the following methods (see the specification of U.S. Pat. No. 5,406,598 and Japanese Non-examined Patent Publication No. 6-201884) to solve the above problem that it is impossible to accurately detect the stability of the reactor core, while the regional oscillation is generated. One to a method of previously selecting reference LPRM signals on the basis of a variance value of a signal, and successively calculating phase differences and amplitude differences between the selected LPRM signals, so as to generate the neutron flux detection signal having a high sensitivity by using the phase differences and the amplitude differences of the LPRM signals as compared with averaging LPRM signals equally. The other is a method of previously estimating a spatial higher mode distribution having the possibility of oscillation, thereby, in a case of averaging the LPRM signals, using the estimated distribution mode as a weighting filter.
The present invention is directed to overcome the foregoing problems. Accordingly, it is one object of the present invention to provided a system for monitoring power of a nuclear reactor, which can accurately monitor an oscillation of the power of the nuclear reactor by using conventional detection signals such as LPRM signals so as to improve the safety of the nuclear reactor and the availability thereof.
Another object of the present invention is to improve the power monitoring method already proposed by the applicants into an online power monitoring method which monitors the power of the nuclear relator in an online state and to provide a system for monitoring the power of the nuclear reactor, which is capable of monitoring an oscillation of the power of the nuclear reactor overlooked in a case of using the conventional power monitoring method.
In order to achieve the such objects and other objects, according to one aspect of the present invention, there is provided a system for monitoring power of a nuclear reactor, comprising: a plurality of neutron flux measuring means arranged in a reactor core of the nuclear reactor for measuring neutron flux so as to generate neutron flux signals on the basis of the measured neutron flux; generating means having a first weighting coefficient and a second weighting coefficient for filtering the neutron flux signals through at least one of the first weighting coefficient and the second weighting coefficient so as to generate a stability monitoring signal, said first weighting coefficient corresponding to a fundamental mode of a neutron flux distribution in the reactor core and said second weighting coefficient corresponding to a higher mode of the neutron flux distribution therein; and stability monitoring means for monitoring a stability of the reactor core according to the stability monitoring signal generated by the generating moans.
In preferred embodiment of this aspect, the reactor core has a substantially homogeneity and wherein said first weighting coefficient and said second weighting coefficient are obtained by using a Bessel function distribution substantially corresponding to a neutron flux distribution of the homogeneous reactor core.
This aspect of the present invention has an arrangement that the generating means is adapted to generate a first stability monitoring signal as the stability monitoring signal by filtering the neutron flux signals through the first weighting. coefficient so as to supply the first stability monitoring signal to the stability monitoring means, and wherein said stability monitoring means has means for gaining a decay ratio related to the first stability monitoring signal, a resonant frequency concerning thereto and an amplitude related thereto and means for evaluating a core stability of the reactor core as the stability thereof according to the decay ratio, the resonant frequency and the amplitude.
This aspect of the present invention has an arrangement that the generating means is adapted to generate a second stability monitoring signal as the stability monitoring signal by filtering the neutron flux signals through the second weighting coefficient so as to supply the second, stability monitoring signal to the stability monitoring means, and wherein said stability monitoring means has means for gaining a decay ratio related to the second stability monitoring signal, a resonant frequency concerning thereto and an amplitude related thereto and means for evaluating a regional stability of the reactor core as the stability thereof on the basis of the decay ratio, the resonant frequency and the amplitude.
Preferred embodiment of this aspect further comprises: measuring and recording means for successively measuring core state data representing a present state in the reactor core according to the neutron flux signals so as to record the core state data; input means for inputting a given operating point of the nuclear reactor having a predetermined operating condition thereof; means for reading out the core state data from the measuring and recording means in response to the input of the given operating point; means for obtaining a thermal-hydraulic prediction condition at the given operating point on the basis of the predetermined operating condition and the read-out core state data; and stability predicting means having a stability analysis model based on a physical model for carrying out a stability analysis by using the thermal-hydraulic prediction condition at the given operating point and the stability analysis model so as to predict a core stability, a channel stability and a regional stability of the reactor core at the given operating point.
In order to achieve the such objects and other. objects, according to another aspect of the present invention, there is provided a system for monitoring power of a nuclear reactor, comprising: a plurality of neutron flux measuring means arranged in a reactor core of the nuclear reactor for measuring neutron flux so as to generate neutron flux signals on the basis of the measured neutron flux, a part of said neutron flux measuring means being disposed to peripheral portions in the reactor core so as to be substantially at equal distance from a center portion of the reactor core; generating means having at least one weighting coefficient for filtering the neutron flux signals through the at least one weighting coefficient so as to generate a stability monitoring signal; and stability monitoring means for monitoring a regional stability of the reactor core according to the stability monitoring signal generated by the generating means.
As is evident from the above descriptions, the system of the present invention can accurately monitor the power oscillation of the nuclear reactor, which is hard to be detected by the conventional APRM signals, and can achieve an improvement in safety and availability factor of the nuclear reactor.